Hard-seededness

Some seeds exhibit 'hard-seededness' - often the result of an impermeable seed coat acting as a barrier to water uptake so that the embryo tissues cannot hydrate.

See graph to observe:

A few untreated seeds may have permeable coats and germinate at most temperatures.

Pretreatment (e.g 'scarification1') stimulates virtually all live seeds to germinate at most temperatures.

Examples: Most temperate and tropical legume tree species.

1Scarification - any 'pretreatment' that removes, abrades, splits, burns or softens the hard seed-coat - without significantly damaging the embryo within; e.g. chipping, filing, boiling / hot water. These are the ONLY tree seeds which do not need to be moist to respond to pretreatment.